1. Field of the Invention
The present invention relates to optically pumped solid-state laser amplifiers. More specifically, the present invention relates to a grating method and apparatus embodiment for reducing or eliminating parasitic oscillations (e.g., amplified spontaneous emission (ASE)) in solid state laser materials.
2. Description of Related Art
Amplified Spontaneous Emission (ASE) strongly impacts performance and efficiency of high-power laser systems. ASE is produced when excited ions in the laser gain medium spontaneously emit light that is amplified by other excited ions. ASE increases the overall decay rate of excited ions and thereby decreases gain and stored energy that is available for amplifying the laser beam. ASE is emitted at all angles within the volume of the gain medium. Most of it is trapped by total internal reflection (TIR), and bounces back and forth between the faces of the gain medium before reaching the periphery. Background information on such deleterious ASE and parasitic oscillation effects can be found in “Fluorescence Amplification and Parasitic Oscillation Limitations in Disk Lasers”, by J. B. Trenholme, NRL Memorandum Rep. 2480, July, 1972; J. E. Swain, et al., J. Appl. Phys., 40, p. 3973 (1969); and J. M. McMahon et al., IEEE J. Quantum Electron. QE-9, p. 992 (1973)).
Much development has gone into methods and materials that are applied to the periphery of the gain medium to out-couple light that reaches this edge (i.e. edge cladding background). However, new solutions are required to allow out-coupling of this light at the faces of the gain media, before it reaches the periphery, in order to allow for optimum energy extraction, larger gain media apertures, and fewer overall gain media for a given energy requirement. In LLNL's NIF laser for example, modeling shows that gain coefficients and stored energy density would be ˜44% greater without ASE. If ASE could be defeated, corresponding fewer laser slabs would be needed to provide the necessary gain, resulting in significant cost savings. Further, the resulting higher gain per slab would enable beamlines to operate at higher intensities without exceeding non-linear phase shift limits.
Another problem concerning slab-geometry laser gain media is the need for higher coupling efficiency of diode pump light into them. For many solid-state lasers, the amplifying slabs or disks absorb only a fraction of the incident pump light due to the limited path length of pump light through the slabs.
In principle, pump-light absorption can be increased by increasing the density of absorbing ions or by increasing the thickness of the laser medium. However, ion density is often limited by material fabrication issues or other factors, such as “concentration quenching” that occurs with Nd3+ ions. Slab thickness is often limited by thermal issues. These limitations can put undesirable constraints on slab and amplifier design and performance.